I have gas fumes when I start the Van from a cold overnight start. on 2005 Pontiac Montana

I never happens once I drive it and restart and also fumes go away after van runs 15 minutes or so. A mechanic suggested I put fuel injector fluid in my gas tank because it could be a gunked up injector. I have ran two full tanks of gas with injector fluid and still get the smell. It is not every time but almost every other.